In a network control unit (NCU) of a communication line used in facsimile equipment (hereinafter referred to as FAX), a modulator-demodulator (hereinafter referred to as the modem), or the like, the presence or absence of a loop current of the line is usually detected by the use of a current sensor circuit, and line control processing or the like is effected on the basis of the result of this detection.
In addition, in not only such FAX and modems but also other types of apparatus using a telephone line adopting, for instance, an LP (loop signaling) system for transmission and receipt of a signal by the opening or closing of a DC loop circuit, a current sensor circuit is provided for the purposes of effecting call processing and cut-off processing with respect to terminal equipment by turning on and off of a DC loop current and for the purposes of effecting call-responding and terminating processing with respect to an exchange by means of the polarity of the loop current.
In this case, as a current sensor circuit, one using a reed relay or one using a photodiode are conventionally used; however, the former type has the problem that reliability is low, while the latter type has the problem that trouble occurs due to a change with time.
Accordingly, for the purpose of preventing the occurrence of such problems, current sensor circuits using a magnetic sensor such as a Hall element are frequently used.
FIG. 2 is a circuit diagram illustrating an example of such a magnetic sensor-type current sensor circuit.
The current sensor circuit shown in this diagram comprises a coil 101 for generating a magnetic flux of a magnitude and a direction that correspond to a value and direction of an input current; a Hall element unit 102 for producing a Hall voltage of a value corresponding to the magnitude and direction of the magnetic flux thus generated by the coil; a drive voltage generating unit 109 for supplying a source voltage to the Hall element unit 102; an inverter/amplifier unit 103 for amplifying the Hall voltage output from the Hall element unit 102; and a compensation voltage generating unit 104 for generating a compensation voltage necessary for effecting offset compensation for the inverter/amplifier unit 103 and unbalance compensation for the Hall element unit 102.
The current sensor circuit further comprises a threshold voltage generating unit 105 for generating a forward threshold voltage and a reverse threshold voltage; a forward comparator unit 106 for generating a "1" signal of a negative logic when the Hall voltage output from the inverter/amplifier unit 103 is higher than a positive-side threshold voltage output from the threshold voltage generating unit 105; and a reverse comparator unit 107 for generating a "1" signal of the negative logic when the Hall voltage output from the inverter/amplifier unit 103 is lower than the reverse threshold voltage output from the threshold voltage generating unit 105.
When a current flows through an electric line in which the coil 101 is inserted, and a magnetic flux of a magnitude corresponding to the value and direction of the aforementioned current is output from the coil 101, the Hall element unit 102 detects the same and generates a Hall voltage of a value corresponding to the value and direction of that magnetic flux. At the same time, the Hall voltage is amplified by the inverter/amplifier unit 103, and a "1" signal of the negative logic is output from the forward comparator unit 106 when the voltage obtained by the amplifying action is higher than the forward threshold voltage. In addition, when the voltage obtained by the amplifying action is lower than the reverse threshold voltage, a "1" signal of the negative logic is output from the reverse comparator unit 107.
In addition to such a current sensor circuit, a circuit which is shown in FIG. 3, for example, is also known. In this drawing, the components that are identical with those shown in FIG. 2 are denoted by the same reference numerals.
The current sensor circuit shown in this drawing is provided with a constant-voltage generating unit 110 instead of the drive voltage generating unit 109 shown in FIG. 2, and a constant voltage is supplied from this constant voltage generating unit 110 to the Hall element unit 102.
However, the following drawbacks have been experienced with these conventional current sensor circuits.
First, with the current sensor circuit shown in FIG. 2, since the drive voltage generating unit 109 is constituted by a resistor 111, although there is the advantage that the circuit can be simplified, when the internal resistance of a Hall element 112 constituting the Hall element unit 102 has changed owing to a change in the temperature of the Hall element 112, the applied voltage also changes correspondingly, causing the value of the Hall voltage to change.
In addition, the current sensor circuit shown in FIG. 3, since the constant voltage generating unit 110 is constituted by a transistor 113 for voltage control and resistors 114, 115 for determining a bias voltage for the transistor 113, it is possible to maintain the voltage applied to the Hall element 112 at a constant level even when the internal resistance of the Hall element 112 has changed owing to a temperature change of the Hall element 112. However, there have been problems in that it is difficult to effect a reduction in cost by a portion in which the number of parts used increases, and that this arrangement is disadvantageous in terms of space.
Furthermore, with the current sensor circuits shown in FIGS. 2 and 3, three resistors 116, 117, 118 are connected in series to constitute the threshold voltage generating unit 105, so that it is impossible to make adjustment of either one of the forward threshold voltage and the reverse threshold voltage output from the threshold voltage generating unit 105. Hence, there has been the drawback that the adjustment operation is difficult.
The present invention has been devised in view of the above-described situation, and its object is to provide a current sensor circuit which allows a reduction in the cost of the circuit and space saving to be effected by reducing the number of parts used, and which is capable of increasing the accuracy of the circuit and simplifying adjustment.